the Great Whin Sill, Northumberland. 249 



twinning less developed ; and as patches showing spherulitic structure. 

 These spherulites have a brownish tinge and form a highlv 

 characteristic component of the inclusions. They have been the last 

 material to consolidate, they are often gathered round a cluster of 

 felspar crystals, or a single individual, and they sometimes show 

 a tendency to assume the outlines of a felspar crystal. 



Quartz is a common constituent of the inclusions, and occurs both 

 in interstitial and idiomorphic forms ; it is most abundant at the 

 edges of the spherulites and is often penetrated by felspar. There 

 are several opaque iron-minerals present. One has a bronze colour 

 and occurs in roundish plates and long slender bands ; another is 

 bluish-black and occasionally shows, in reflected light, a structure 

 like that of the films. Sections cut vertically through the films often 

 show these two minerals, side by side, sometimes fringed with a third, 

 dull, non-reflecting mineral ; and when the section makes a low 

 angle with the film the dull mineral is seen to be entirely in the 

 skeletal condition. The films are thus sometimes composite, and 

 appear to consist of pyrrhotite and magnetite, possibly with a little 

 titaniferous oxide, and these minerals also occur irregularly distributed 

 throughout the inclusions. In addition to these some of the slides 

 also show a few scattered crystals of ilmenite which exhibit the 

 characteristic cinnamon-brown colour. 



Apatite is the only accessory mineral identified with certainty. 

 It occurs, with moderate frequency, in well-developed crystals with 

 dark centres, and also in the form of bundles of long, but exceedingly 

 slender, needles. These are very abundant in all the sections, and 

 undoubtedly contribute greatly to the high phosphorus-content 

 established by analysis. They penetrate all the other constituents 

 except the iron-minerals. 



Most of the slides show small amygdules, fringed with iron-oxide 

 and filled with secondary quartz and calcite. This confirms the 

 observations made on hand-specimens as to the frequency of 

 occurrence of amygdules in the inclusions. Another observation, 

 mentioned above, concerning the contact of the films, finds support 

 from the study of the sections ; for it is found that adjacent films 

 sometimes meet at a sharp angle, and are sometimes separated for 

 some little distance, the intervening space being occupied by the 

 materials peculiar to the inclusions. 



With respect to the relations between the inclusions and the 

 surrounding basalt, the study of the slices yields some rather 

 unexpected results. Though the petrological contrast of the rocks 

 is considerable, the contact is never clearly marked. The general 

 shape of the inclusions in the sections is circular, but the edges 

 are somewhat Avavy in outline, owing to interpenetration of the 

 components of each rock-species. Generally speaking, it may be 

 stated that the lower the magnification the better is the definition 

 of the inclusions. 



Again, though in hand-specimens half of an inclusion is quite 

 sharply defined by the films, the latter never constitute boundaries in 

 the sections; there is practically always some felspathic material on 

 the outside of the films, even though the thickness of this is, at times, 



